Dispersion powders are polymer powders obtainable by spray-drying the corresponding aqueous polymer dispersions. A distinction is made here between dispersion powders which are redispersible in water and those which do not have this property. Water-redispersible polymer powders are generally obtained by drying the corresponding aqueous polymer dispersions in the presence of a drying aid (generally a protective colloid) and of antiblocking agent. Because of the protective colloid content, irreversible adhesion of the polymer particles is firstly prevented in the course of the drying operation, since the polymer particles are encased by the water-soluble protective colloid particles. Secondly, this protective colloid matrix, which redissolves when the polymer powder is dispersed in water, has the effect that the polymer particles are again present in the aqueous redispersion with the particle size of the starting dispersion.
To minimize powder blocking, especially in the case of dispersion powders based on polymers having a glass transition temperature Tg of <25° C., and to improve the free flow and flowability, antiblocking agents (anticaking agents) are added to the dispersion powders in the course of production thereof.
The antiblocking agent (ABA) serves to ensure free flow and storage stability of the polymer powder obtained after spray drying (dispersion powder). Antiblocking agents are an essential constituent of the dispersion powder especially when blocking of the powder or caking on the dryer wall are to be expected because of a low glass transition temperature of the polymer or high hygroscopicity. The powdering of the particle surface with antiblocking agent, because of relatively low agglomerate formation, more particularly, also reduces the proportion of coarse particles, which has a very advantageous effect on the overall drying operation and on the properties of the powder (for example free flow and storage stability).
Dispersion powders of this kind are used in many kinds of applications, including in coating compositions and in adhesives for a wide variety of different substrates. One example is use as a binding powder for particulate natural materials (VinnexR powders from Wacker Chemie AG). The use thereof in chemical products for construction is widespread, frequently in conjunction with mineral binders (VinnapasR powders from Wacker Chemie AG). Examples thereof are construction adhesives, especially tile adhesives, renders and mortar compounds, paints, spackling compounds, leveling compounds, thermal insulation composite systems and jointing mortars. The advantage of the water-redispersible dispersion powders lies particularly in the option of using them in prefabricated, storable dry mixes, optionally together with mineral binders such as cement, and of making them ready for use only immediately before use by addition of water. Dry mixes of this kind can be more easily transported (without a water content) compared to systems in paste form and offer advantages in storage, for example insensitivity to frost and resistance to microbe infestation.
The dispersion powders are produced in known spray dryers, by atomizing the polymer dispersion (feed), generally at the upper end of the dryer, by means of a one-phase nozzle under pressure, or atomizing it together with compressed air using a two-phase nozzle or multiphase nozzle, or atomizing it by means of a rotary atomizer. For spray-drying by means of a rotary atomizer, EP 690278 A1 proposes introducing the antiblocking agent with the conveying gas below the atomizer disk of a rotary atomization dryer. For this purpose, the antiblocking agent is conducted to the rotary atomizer with conveying air in a conveying line which runs transversely through the dryer. This construction has the disadvantage that the transverse pipe is very prone to soiling and the rigid construction hinders the operation of the dryer. For example, this construction prevents the use of a clearing blower to clean the dryer walls.
The problem addressed was that of providing a process for spray-drying aqueous polymer dispersions with addition of antiblocking agent by means of a rotary atomizer, by means of which very substantially homogeneous and complete coverage of the polymer particles with antiblocking agent is obtained.